When klebsiella aerogenes were grown and then transferred to medium containing sufficient chloramphenicol to block protein synthesis, degradation of glutamine synthetase occurred. Inactivation of enzyme was stimulated by glucose or glycerol, and inhibited by dinitrophenol. Inactivation occurred more rapidly at higher pH values, over a range from pH 6 to 8. Inactivation also occurred more rapidly in cells originally harvested during log phase of growth than in cells incubated for several hours in stationary phase prior to harvest for resuspension and incubation in the presence of chloramphenicol. Cell-free extracts have been prepared which inactivate glutamine synthetase at a rate similar to that observed in intact cells incubated in the presence of chloramphenicol. As observed in intact cells, enzyme inactivation in the extracts occurred more rapidly at higher pH values, and in extracts prepared from cells originally harvested during log phase of growth. Enzyme inactivation in the extracts was stimulated by some metal ions and inhibited by others. EDTA inhibited inactivation. Klebsiella glutamine synthetase has been purified to homogeneity or near homogeneity. The purified enzyme incubated alone was stable under conditions in which it was inactivated when added to a cell extract. A boiled extract was unable to inactivate the purified enzyme.